Constructing a rack that consists of two pieces of wood or metal perpendicular to the garage wall is a good way to get metal chairs out of the way. The rack supports the folded chairs underneath their backrests and allows them to hang. With the chairs folded, you can fit a large number of them on a rack. For more chairs, build multiple racks, installing them as high as possible off the floor to keep the chairs out of the way.
You can store metal chairs in the rafters of your garage if the ceiling joists are wide enough to fit the chairs between them. Screw wooden slats across the bottoms of several joists, perpendicular to the joists. By screwing several slats across the joists so the slats are 12 or 14 inches apart, you create an area where the chairs can be stored by placing them between the joists and on top of the slats. If you have 12-inch-high joists, you can probably fit three folded chairs into each space between them.
The easiest way to store metal chairs in your garage is to stack them in the corner. Choose the most out-of-the-way corner in your garage and clear out anything that is used more often than the chairs. You don't want them to block your lawnmower or other yard essentials, for example. Stack the chairs against a wall and put an old sheet over them to keep them from getting covered with dust or dirt. This will save you cleaning time when you want to use the chairs.